1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor layer formed over an insulating layer, and to a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, an integrated circuit has been developed in which an SOI (silicon on insulator) substrate is used instead of a bulk silicon wafer. By taking advantage of properties of a thin single crystal silicon layer formed over an insulating layer, a semiconductor layer in transistors in an integrated circuit can be separated completely, and the transistors can be a complete depletion type; thus, a semiconductor integrated circuit with high added value such as high integration, high-speed driving, or low power consumption can be realized.
As an SOI substrate, a SIMOX substrate and a bonding substrate are known. For example, in a SIMOX substrate, an SOI structure is achieved by implanting oxygen ions into a single crystal silicon substrate and performing a thermal treatment at 1300° C. or more to form a buried oxide (BOX) film, thereby forming a thin single crystal silicon film at a surface of the single crystal silicon substrate.
In a bonding substrate, two single crystal silicon substrates (a base substrate and a bond substrate) are bonded to each other with an oxide film interposed therebetween, and one of the single crystal silicon substrates (the bond substrate) is thinned from a rear face (a face which is not bonded), whereby a thin single crystal silicon film is formed to obtain an SOI structure. Since it is difficult to form a thin and uniform single crystal silicon film by grinding or polishing, a technique utilizing hydrogen ion implantation called the “Smart Cut” (registered trademark) is proposed (for example, see Reference 1: Japanese Published Patent Application No. H5-211128).
A summary of the method for manufacturing an SOI substrate is as follows: hydrogen ions are implanted into a single crystal silicon substrate by an ion implantation method, thereby forming an embrittlement layer at a given depth from a surface of the single crystal silicon substrate; next, another single crystal silicon substrate, which functions as a base substrate, is oxidized to form a silicon oxide film; after that, the single crystal silicon substrate into which the hydrogen ions are implanted and the silicon oxide film of the single crystal silicon substrate which functions as the base substrate are bonded to each other, so that the two single crystal silicon substrates are bonded to each other; subsequently, the single crystal silicon substrate is separated along the embrittlement layer through a thermal treatment, whereby a substrate is formed in which a thin single crystal silicon layer is bonded to the single crystal silicon substrate which functions as the base substrate.
The ion implantation method is also called an “ion introduction method”; in the ion implantation method, particles to be implanted into a sample are ionized in a vacuum and are accelerated with direct current or high-frequency waves, to be implanted into the sample. An ion implantation apparatus utilizing an ion implantation method includes an ion source, a mass separation portion, an acceleration portion, a beam scan portion (electrostatic scan), an implantation chamber (an end station), and a vacuum evacuation unit. Since an ion beam has a nonuniform cross section, a sample is scanned electrically with an ion beam so that the ion beam can be uniform on a surface of the sample. Implanted particles exhibit Gaussian distribution in a depth direction.
Further, an example of a semiconductor device utilizing an SOI substrate is disclosed in which hydrogen is implanted by an ion implantation method (see Reference 2: Japanese Published Patent Application No. 2000-012864).
In a process of bonding two single crystal silicon substrates, however, if minute foreign matters (e.g., particles) attach to a bonding interface between the single crystal silicon substrates, a large number of voids are generated, which increases bonding defects.
Further, if water molecules at the bonding interface between the single crystal silicon substrates are not diffused or absorbed into surroundings, water accumulates at the bonding interface, which increases bonding defects. Furthermore, a dehydration condensation reaction does not proceed at the bonding interface, so that a strong bond is not formed and there occurs a bonding defect.
If there frequently occur such bonding defects, the bonding strength decreases and a thin single crystal silicon layer can come to be unstuck at the bonding interface between the single crystal silicon substrate which functions as the base substrate and the thin single crystal silicon layer which has been separated. Further, also in manufacturing a semiconductor device, such bonding defects hinder manufacture of a semiconductor device having high quality.